Acyrologia (n.): (Rhetoric) the imprecise use of language, failure to use the proper term.
This word, which I came upon in the commentary on Donatus’ Barbarismus attributed to Robert Kilwardby (as translated in the recent rhetorical door-stopper edited by Rita Copeland and Ineke Sluiter), is a treasure and a joy forever. In the relevant passage, the commentator explains that an allegorical intention can excuse acyrologia. That is, if an instance of improper language use is “perpetrated on purpose for special effect” it becomes a species of allegory, and is thus no longer inappropriate.
We’ve all found plenty of example of this in student work: I can think of one that made me chuckle while marking exam papers earlier this year in which I was assured that life for “pheasants” was particularly difficult in medieval times. Poor fowls. Not sure why they had it so tough… But for some reason, more than student malapropisms, this made me think of my grandfather, one of whose favourite sayings was “looks like an obstacle illusion!” I’m pretty sure this would have constituted acyrologia if it hadn’t been intended for humorous effect… Good one, Gramps!
 Medieval Grammar and Rhetoric: Language Arts and Literary Theory, A.D. 300–1475, ed. by Rita Copeland and Ineke Sluiter (Oxford: Oxford University Press, 2009), (p. 734). The quote comes from the editors’ definition and discussion of the term at n. 54.